The invention relates to a method for converting a video signal in one of an analogue and digital form to the other of the analogue and digital form with enhanced signal to noise ratio, so that a relatively low order analogue filter is suitable for filtering the analogue form of the signal.
In the conversion of video signals from digital form to analogue form because of the relatively wide band width of video signals, it is desirable that the analogue reconstruction filter be of the lowest possible order while at the same time providing a good quality analogue video signal. Analogue reconstruction filters of relatively high order tend to be complex and thus relatively expensive, particularly when the band width of the signal is relatively wide. Video signals, in general, are required in three formats, namely, standard definition format, progressive scan format and high definition format. The band width of the video signals in the three formats range from 6 MHz for standard definition format to 30 MHz for high definition format. Progressive scan format signals are of band width of 12 MHz. These band widths of video signals are relatively high, particularly when compared with audio signals which are of band width in the KHz range.
In order to convert such audio signals from digital form to analogue form and to provide the analogue form with an acceptable signal to noise ratio, it is known to over-sample the digital form of such audio signals at high over-sampling rates of the order of one hundred and twenty times the nyquist sampling frequency. In order to achieve a desired signal to noise ratio at such high over-sampling rates, the over-sampled signals are subjected to sigma-delta modulation in a sigma-delta modulator for noise shaping quantisation noise resulting from over-sampling the signals. Because of the high over-sampling rate the sigma-delta modulator must be of relatively high order, which may be as high as seventh order. However, because of the relatively narrow band width of such signals, the output of the sigma-delta modulator may be a single bit output, which thus requires that the digital to analogue converter (DAC) need similarly be only a single bit DAC. The analogue reconstruction filter, however, must be one order higher than the order of the sigma-delta modulator, in order to cancel the shaped quantisation noise. While in the case of audio signals a relatively high order analogue reconstruction filter is required where the audio signal is subjected to sigma-delta modulation of relatively high order, since the band width of the audio signal is relatively narrow, such a high order reconstruction filter can be tolerated. In general, the complexity and cost of such reconstruction filters of the appropriate order is relatively low due to the narrow band width of the signals. However, because of the wide band width of video signals, an analogue reconstruction filter of such high order would be entirely unacceptable.
There is therefore a need for a method for converting a video signal in digital form to analogue fort which permits the use of a relatively low order analogue reconstruction filter, and indeed, there is also a need for a method for converting video signals in either one of an analogue and digital form to the other of the analogue and digital form which likewise permits the use of a relatively low order analogue filter.
The present invention is directed towards providing such a method.
According to the invention there is provided a method for converting a video signal in one of an analogue and digital form to the other of the analogue and digital form, the video signal in the digital form being a multi-bit signal, the method comprising the steps of:
over-sampling the video signal at a low over-sampling rate,
subjecting the over-sampled video signal to sigma-delta modulation in a sigma-delta modulator of low order for noise shaping the over-sampled signal for enhancing the signal to noise ratio of the video signal, and
filtering the analogue form of the video signal in an analogue filter of one order greater than the order of the sigma-delta modulator.
Preferably, the order of the sigma-delta modulator does not exceed four. Advantageously, the order of the sigma-delta modulator does not exceed three. Ideally, the sigma-delta modulator is a second order sigma-delta modulator.
In one embodiment of the invention the sigma-delta modulation of the over-sampled video signal is carried out in a multi-bit sigma-delta modulator.
In another embodiment of the invention the over-sampling rate at which the video signal is over-sampled does not exceed eight times the nyquist sampling frequency.
In one embodiment of the invention the video signal is in standard definition format and the over-sampling rate is eight times the nyquist sampling frequency.
In another embodiment of the invention the video signal is in progressive scan format and the over-sampling rate is four times the nyquist sampling frequency.
In a further embodiment of the invention the video signal is in high definition format and the over-sampling rate is twice the nyquist sampling frequency.
In one embodiment of the invention the video signal after being over-sampled is at least an eight bit signal.
In another embodiment of the invention the video signal after being over-sampled is at least a ten bit signal.
In a further embodiment of the invention the video signal after being over-sampled is at least a twelve bit signal, while in a still further embodiment of the invention the video signal after being over-sampled is at least a fourteen bit signal.
In another embodiment of the invention the noise shaped signal after sigma-delta modulation is subjected to bit shuffling for minimising mismatch noise generated in the conversion of the signal from one of the analogue and digital forms to the other of the analogue and digital forms.
In one embodiment of the invention the video signal is a digital video signal, and is converted to analogue form.
In another embodiment of the invention the analogue filter is a reconstruction filter.
Advantages of the Invention
The advantages of the invention are many. A particularly important advantage of the invention is that the method according to the invention for converting video signals in one of analogue and digital form to the other of analogue and digital form permits the use of a relatively low order analogue filter for filtering the analogue form of the signals. When the method is used for converting digital video data signals to analogue form, a relatively low order Butterworth analogue reconstruction filter can be used, and typically, the analogue reconstruction filter can be a third or fourth order filter. This allows a relatively simple and inexpensive analogue reconstruction filter to be used. A particularly important advantage of being able to use a relatively low order analogue reconstruction filter is that in general, a low order reconstruction filter gives a more linear phase response, which is important for the quality of video signals.
When the method according to the invention is applied to the conversion of video signals in analogue form to digital form, similar advantages arise in that an analogue anti-aliasing filter for filtering the analogue form of the signal can likewise be provided as a relatively low order filter, typically, a third or fourth order filter, thus permitting the use of a relatively simple and inexpensive analogue filter.
These advantages and others will be readily apparent to those skilled in the art from the following description of some embodiments of the invention, which are given by way of example only, with reference to the accompanying drawings.